The present invention relates generally to tools used in subterranean wells and, in a preferred embodiment thereof, more particularly provides a slurry delivery apparatus for use in formation fracturing operations.
Oftentimes, a potentially productive geological formation beneath the earth's surface contains a sufficient volume of valuable fluids, such as hydrocarbons, but also has a very low permeability. "Permeability" is a term used to describe that quality of a geological formation which enables fluids to move about in the formation. All potentially productive formations have pores, a quality described using the term "porosity", within which the valuable fluids are contained. If, however, the pores are not interconnected, the fluids cannot move about and, thus, cannot be brought to the earth's surface.
When such a formation having very low permeability, but a sufficient quantity of valuable fluids in its pores, is desired to be produced, it becomes necessary to artificially increase the formarion's permeability. This is typically accomplished by "fracturing" the formation, a practice which is well known in the art and for which purpose many methods have been conceived. Basically, fracturing is achieved by applying sufficient pressure to the formation to cause the formation to crack or fracture, hence the name. The desired result being that the cracks interconnect the formarion's pores and allow the valuable fluids to be brought out of the formation and to the surface.
A conventional method of fracturing a formation begins with drilling a subterranean well into the formation and cementing a protective tubular casing within the well. The casing is then perforated to provide fluid communication between the formation and the interior of the casing which extends to the surface. A packer is set in the casing to isolate the formation from the rest of the wellbore, and hydraulic pressure is applied to the formation via tubing which extends from the packer to pumps on the surface.
The pumps apply the hydraulic pressure by pumping fracturing fluid down the tubing, through the packer, into the wellbore below the packer, through the perforations, and finally, into the formation. The pressure is increased until the desired quality and quantity of cracks is achieved and maintained. Much research has gone into discerning the precise amount and rate of fracturing fluid and hydraulic pressure to apply to the formation to achieve the desired quality and quantity of cracks.
The fracturing fluid's composition is far from a simple matter itself. Modern fracturing fluids may include sophisticated manmade proppants suspended in gels. "Proppant" is the term used to describe material in the fracturing fluid which enters the formation cracks once formed and while the hydraulic pressure is still being applied (that is, while the cracks are still being held open by the hydraulic pressure), and acts to prop the cracks open. When the hydraulic pressure is removed, the proppant keeps the cracks from closing completely. The proppant thus helps to maintain the artificial permeability of the formation after the fracturing job is over. Fracturing fluid containing suspended proppant is also called a slurry.
A proppant may be nothing more than a very fine sand, or it may be a material specifically engineered for the job of holding formation cracks open. Whatever its composition, the proppant must be very hard and strong to withstand the forces trying to close the formation cracks. These qualities also make the proppant a very good abrasive. It is not uncommon for holes to be formed in the protective casing, tubing, pumps, and any other equipment through which a slurry is pumped.
Particularly susceptible to abrasion wear from pumped slurry is any piece of equipment in which the slurry must make a sudden or significant change in direction. The slurry, being governed by the laws of physics, including the principles of inertia, tends to maintain its velocity and direction of flow, and resists any change thereof. An object in the flowpath of the slurry which tends to change the velocity or direction of the slurry's flow will soon be worn away as the proppant in the slurry incessantly impinges upon the object.
Of particular concern in this regard is the piece of equipment attached to the tubing extending below the packer which takes the slurry as it is pumped down the tubing and redirects it radially outward so that it exits the tubing and enters the formation through the perforations. That piece of equipment is called a crossover. Assuming, for purposes of convenience, that the tubing extends vertically through the wellbore, and that the formation is generally horizontal, the crossover must change the direction of the slurry by ninety degrees. Because of this significant change of direction, few pieces of equipment (with the notable exception of the pumps) must withstand as much potential abrasive wear as the crossover.
In addition, the crossover is frequently called upon to do several other tasks while the slurry is being pumped through it. For example, the crossover typically contains longitudinal circulation ports through which fracturing fluids that are not received into the formation after exiting the crossover are transmitted back to the surface. Space limitations in the wellbore dictate that the circulation ports are not far removed from the flowpath of the slurry through the crossover. If the crossover is worn away such that the slurry flowpath achieves fluid communication with the circulation ports in the crossover, the fracturing job must cease. Once stopped, the frac job cannot be recommenced or completed. Hence, it is very important that the crossover does not fail while the job is in process. If the frac job is not halted after the crossover fails, the slurry will enter the circulation ports in the crossover and travel back to the surface without delivering the proppant to the formation.
For the above reasons and others, the crossover has commonly been considered a disposable piece of equipment, usable for only one fracturing job, or worse, less than one fracturing job. Even when it survives a fracturing job, it is usually sufficiently worn that no further use may be made of it. This is unfortunate because the crossover is also typically one of the most expensive pieces of equipment used in a fracturing job due to its high machining and material costs.
Further, customers are now demanding fracturing jobs with high flow rates, high pressures, higher quantities, and higher density proppants. All of these increase wear on the crossover and thereby increase the likelihood of crossover failure during the fracturing job.
Attempts have been made to provide a solution for these problems. One involves making the crossover out of extremely hard and abrasion wear resistant materials. This has proven to reduce the rate of abrasion wear of the crossover. It is, however, enormously expensive to make an entire crossover out of a sufficiently wear resistant material. No economic advantage is actually achieved by this solution over the disposable crossover made of less wear resistant, but much less expensive, materials.
Another proposed solution is to utilize surface treatment of less expensive alloy steels to achieve a wear resistant crossover surface. Methods such as carburizing, nitriding, etc., which produce a high surface hardness do indeed slow the abrasion wear rate of the crossover at less expense than using exotic materials. However, as soon as the hardened surface layer has been breached, the crossover begins to wear away rapidly. For this reason, surface-hardened crossovers are also not sufficiently durable for the newer high flow, high pressure fracturing jobs. The extra expense of surface-hardening a disposable crossover makes this solution uneconomical as well.
Another area of concern in regard to abrasion wear during fracturing jobs is the protective casing lining the wellbore. Since the crossover typically directs the slurry flow radially outward, the casing is directly in the altered slurry flowpath. Unintended, misplaced holes in the casing are to be avoided, since it is the casing which provides the only conduit extending to the surface through which all other conduits and equipment must pass.
From the foregoing, it can be seen that it would be quite desirable to provide a slurry delivery apparatus which does not have the economic disadvantages of the solutions enumerated above, but which allows repeated use thereof. It would also be desirable to provide a slurry delivery apparatus which minimizes the abrasive wear of the casing during fracturing operations. It is accordingly an object of the present invention to provide such a slurry delivery apparatus and associated methods of using same.